Prior to this invention it has been known to make adhesives and coating compositions using coal tar pitch and certain ethylene polymers such as copolymers of ethylene with acrylic acid and copolymers of ethylene with vinyl acetate. Such coatings and adhesives have been generally successful, but they suffer from a significant defect in that the creep resistance is unsatisfactorily low.
Creep resistance is a dimensional change which is caused under loads and is a function of time and temperature. Although in some applications a certain amount of creep is tolerable or even useful, in most applications the creep effect is undesirable since the coating or adhesive is gradually weakened and the bond is ultimately destroyed. My invention relates to the use of a particular cross-linking agent in the pitch/ethylene copolymer composition.
Cross-linking of polymers is not new. For example, in U.K. Pat. 1087286, the cross-linking of ethylene/acrylic acid copolymers and ethylene/vinyl acetate copolymers is described. In particular, the use of dicumyl peroxide is disclosed in the fourth line of the top of Page 3. See also French Pat. No. 1488913.
Pitch derived from coal tar is used in combination with various polymers including ethylene copolymers in U.S. Pat. Nos. 3,267,083, 3,309,329, 3,392,132, 3,402,742, and 3,318,977. In addition, the reader may be interested in the following U.S. Pat. Nos. 3,402,223, 3,410,928, 3,425,981, 3,503,914, 3,577,365, and 3,654,207.
So far as I am aware, no one has proposed to incorporate or has incorporated into a single composition a coal tar pitch, ethylene/vinyl acetate copolymers and ethylene/acrylic acid copolymers (EVA and EAA) including a crosslinking agent such as dicymyl peroxide, nor has anyone previous to this invention disclosed that the use of dicumyl peroxide has a cross-linking agent will not only cross-link the polymers used in such compositions but will greatly inhibit the tendency to creep.